In recent years, there has been a demand not only for improvement in strength of steel sheets used in automobiles but also for improvement in impact resistance thereof, in order to enhance collision safety while realizing a weight reduction of automobiles.
As a high-strength steel sheet having large collision absorbing energy, Patent Document 1 describes a high-strength steel sheet containing, in weight %, C: 0.05 to 0.3%, Si: 2.0% or less, Al: 0.01 to 2.0%, Mn: 0.5 to 4.0%, Ni: 0 to 5.0%, P: 0.1% or less, S: 0.1% or less, and N: 0.01% or less, with the balance being Fe and inevitable impurities, and having a chemical composition satisfying 1.5−3.0×C≤Si+Al≤3.5−5.0×C and Mn+(Ni/3)≥1.0(%), wherein a bake hardening amount of the steel sheet is 50 MPa or more.
Further, as a high-tension steel sheet excellent in collision absorbency, Patent Document 2 describes a high-ductility, high-tension steel sheet which has a steel structure including: bainite having a volume fraction VB given by an expression VB≤(TSs/60)−1 (TSs: tensile strength (MPa) in a static tensile test); and retained austenite whose C content is 1.2 mass % or less and whose volume fraction is 5% or more, with the balance being ferrite, wherein a yield ratio in the static tensile test is 0.6 or more, and a static-dynamic ratio of the steel sheet is high, with a ratio TSd/TSs between tensile strength in a dynamic tensile test and tensile strength in the static tensile test satisfying a relation given by an expression TSd/TSs≥0.8+(300/TSs) (Tsd: the tensile strength (MPa) in the dynamic tensile test at a 1000/s strain rate).
Further, as a method of manufacturing a high-strength cold-rolled steel sheet excellent in impact property, Patent Document 3 describes a manufacturing method including: hot-rolling a slab which has a composition containing C: 0.08 to 0.18 mass %, Si: 1.00 to 2.0 mass %, Mn: 1.5 to 3.0 mass %, P: 0.03 mass % or less, S: 0.005% mass % or less, and T.Al: 0.01 to 0.1 mass % and in which a Mn segregation degree defined by an expression (Mn segregation degree=(a Mn concentration at a center portion of the slab−a Mn concentration at a base)/the Mn concentration at the base) is 1.05 to 1.10; after cold-rolling, performing heating in a two-phase region or a single-phase region of 750 to 870° C. for a 60 second retention time or longer on a continuous annealing line; thereafter, after cooling in a 720 to 600° C. temperature region at a 10° C./s average cooling rate or less, performing cooling to 350 to 460° C. at a 10° C./s average cooling rate or more to keep this temperature for 30 seconds to 20 minutes, and thereafter performing cooling to room temperature to produce a five-phase structure of polygonal ferrite+acicular ferrite+bainite+retained austenite+martensite.
As a steel sheet used as a steel sheet for automobiles, Patent Document 4 describes an alloyed hot-dip galvanized steel sheet containing, in mass %, C: 0.05 to 0.25%, Si: 0.5% or less, Mn: 1 to 3%, P: 0.1% or less, S: 0.01% or less, Al: 0.1 to 2%, and N: less than 0.005%, with the balance being Fe and inevitable impurities, wherein Si+Al≥0.6%, (0.0006Al) %≤N≤0.0058%−(0.0026×Al) %, and Al≤(1.25×C0.5−0.57 Si+0.625 Mn) % are satisfied.
As a high-strength alloyed hot-dip galvanized steel sheet excellent in energy absorbency, Patent Document 5 describes one whose base material is a steel sheet having: a component composition containing C: 0.05 to 0.20 mass %, Si: 0.3 to 1.5 mass %, Mn: 1.0 to 2.5 mass %, and P: 0.1 mass % or less, with the balance being Fe and inevitable impurities; and a microstructure containing one or two out of martensite and retained austenite totally in 25 to 50 vol %, with the balance being ferrite and bainite, wherein alloying hot-dip galvanization is applied to both surfaces of the steel sheet.
As a high-ductility, high-tension cold-rolled steel sheet excellent in surface property and impact absorbency, Patent Document 6 describes one containing, in weight ratio, C: 0.06 to 0.25%, Si: 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%, Ti: 0.003 to 0.08%, and N: 0.01% or less, with the balance being Fe and inevitable impurities, wherein a Ti content satisfies a relation of (48/14)N≤Ti≤(48/14)N+(48/32)S+0.01, and a structure after cold rolling-recrystallization annealing is a structure containing 5% retained austenite or more in volume fraction.
As a high-ductility, high-strength steel sheet excellent in low-temperature toughness, Patent Document 7 describes one having a structure which contains, in area %, 60% bainite or more and 1 to 20% retained γ, with the balance being substantially ferrite, wherein the retained γ exists in a grain of the bainite.